Magnetic recording hard disk drives use a servo-mechanical positioning system to move the read/write heads from one data track to another data track and to hold the heads on the tracks as required for read and write operations. Current hard disk drives use a rotary voice-coil-motor (VCM) as the single or primary actuator to position the read/write heads on the data tracks. Typically, each read/write head is attached to the end of a head carrier or air-bearing slider that rides on a cushion or bearing of air above the rotating disk. The slider is attached to a relatively flexible suspension that permits the slider to “pitch” and “roll” on the air bearing, with the suspension being attached to the end of the VCM actuator arm.
As magnetic recording technology improves, the data tracks are decreasing in width to accommodate more data in less space. This increase in track density increases the requirements on the servo control system to maintain the heads on the tracks in the presence of internal and external disturbances. This generally requires an increase in the servo bandwidth, accompanied with an increase in open-loop gain at the frequencies below the servo bandwidth. However, mechanical resonances limit the achievable servo bandwidth with the single-stage VCM actuator.
To address this problem, disk drives with dual-stage actuators have been proposed. Various types of secondary actuators, such as piezoelectric and electrostatic milliactuators and microactuators, have been proposed for location on the VCM actuator arm for driving the suspension, on the suspension between the suspension and the slider for driving the slider, or on the slider for driving just the read/write head. Generally, in the servo control scheme for a disk drive with a dual-stage actuator, the VCM is responsible for large-amplitude, low-frequency motion of the heads and the secondary actuator is responsible for small-amplitude, high-frequency motion of the heads. Because of the constraints on the location, size, mass, and frequency range of these secondary actuators, they have relatively small ranges of motion, on the order of a few microns. These limited ranges of motion of the secondary actuators impose constraints on the dual-stage servo controller design.
Generally, the design of a dual-stage controller starts with the VCM controller, typically with a design that is very similar to a single-stage VCM controller. In particular, the stability of the VCM is assured with adequate robustness or stability margins as if it were to operate without the secondary actuator. Then the controller for the secondary actuator is designed to achieve the desired combined dual-stage bandwidth. The secondary actuator control loop and the combined dual-stage control loop are also designed to ensure adequate stability separately and jointly with the other control loops. This process is satisfactory for limited increases in the bandwidth above what is achievable with only the VCM.
As the bandwidth is pushed to even higher frequencies, the limited stroke of the secondary actuator and the stability limits of the VCM control loop start to impact the overall servo design. Specifically, the low frequency gain of the combined open-loop frequency response may be lower than required for the higher bandwidth system. The primary actuator cannot be used to increase the low frequency gain because of the stability limitations associated with its mechanical resonances. The secondary actuator cannot be used to increase the low frequency gain due to its stroke limitations.
What is needed is a magnetic recording disk drive with a dual-stage actuator and a servo control system that has an open-loop low-frequency gain increase over single-stage designs comparable to the open-loop mid-to-high-frequency gain increase normally associated with dual-stage actuator designs.